An ultrasound system has been extensively used in the medical field due to its non-invasive and non-destructive nature. Modern high-performance ultrasound imaging diagnostic systems and techniques are commonly used to produce two-dimensional or three-dimensional ultrasound images of internal features of patients.
The ultrasound system transmits ultrasound signals to the target objects, receives echo signals reflected from the target objects and provides a color Doppler mode image of the target object based on the echo signals. In the color Doppler mode image, velocities of the target object (e.g., blood flow), which flows toward an ultrasound probe, are indicated in a first color (e.g., red), while velocities of the target object, which flows away from an ultrasound probe, are indicated in a second color (e.g., blue).
The signal received by the ultrasound probe is converted into a digital signal at a front end. The digital signal is receive-focused to thereby obtain a receive-focused signal. The band of the receive-focused signal is modulated by a mixer and then the receive-focused signal is appropriately decimated so that In-phase/Quadrature (IQ) signal is obtained. This IQ signal is referred to as a baseband IQ signal. The IQ signal may be expressed by the following equation.XIQ=C+F+N  (1)wherein XIQ represents an IQ signal, C represents a clutter signal generated from a tissue, F represents a blood flow signal generated from blood flow, and N represents a noise signal generated from the system and exterior.
Essential information from Equation (1) is information on F, i.e., the blood flow signal. Color Doppler processing represents a process of extracting components on the blood flow signal from the IQ signal and processing the extracted components into data for display on a screen. Clutter filtering is an essential processing step in the color Doppler processing. The clutter filtering is performed to remove a low-band clutter signal from a Doppler signal to thereby extract only a blood flow signal and a noise signal. The filtered Doppler signal contains the blood flow signal and the noise, which are mixed with each other. The noise may be removed through signal processing. Then, the components on the blood flow signal are displayed on a screen.
Generally, the clutter signal is distributed at a low frequency band, whereas the blood flow signal is distributed at a high frequency band. Thus, a high pass filter should be designed to remove the clutter signal. However, the amplitude of the clutter signal is higher than that of the blood flow signal by 40-60 dB. As such, it is not easy to extract only the blood flow signal. Accordingly, a high performance of the high pass filter is required.
Further, since an effective clutter filtering may be limited due to a small ensemble number, various filtering techniques have been developed. The clutter filter may be classified into a finite impulse response (FIR), an infinite impulse response (IIR) and a Regression. In case of the IIR filter, various initializing techniques such as zero, step, exponential, projection, etc. have been developed to suppress the transient. The Regression-type filter is classified into a polynomial filter, a sinusoidal regression filter and the like.
Conventionally, a clutter filter is properly selected depending on the application of an ultrasound system and the clutter filtering is performed with only the selected clutter filter. In such a case, i.e., if an identical clutter filter is applied to entire regions within a region of interest, the quality of an ultrasound image may be degraded.